Standardize the statements format with jupylates

• Add an item "action": "execute"
• [ ] Use an item "activity": "..." to specify the notebook name.
• How to store the cell id? Is that part of the activity? Would we want to have a notion of "subactivity"?
• [ ] Add an item "student": "???", initialized with the user name as returned by the system?
• What to do if a cell metadata is not present? Store it as null, "undefined", or not store it at all? Presumably the latter (!6 (merged) ).